This invention relates generally to a rotor for an electrical energy conversion transducer such as a generator or electric motor. More particularly, this invention pertains to a rotor assembly having permanent magnets attached thereto and is well suited for use with an electric permanent magnet generator.
Electric motors and generators utilize a rotor which rotates within a cage, the cage having stationary wire windings and comprising a stator. For example, a generator may have a rotor with permanent magnets mounted about the rotor circumference. This type of generator rotor produces a rotating magnetic field, and the generated current is taken directly from the stator. Similarly, certain motors may also utilize a permanent magnet rotor and a stator whose coils are selectively energized by a power source to cause the rotor to turn.
One well known rotor construction has a one-piece steel hub having a cylindrical outer periphery machined to provide the desired number and spacing of flat surfaces upon which permanent magnets are mounted. Each magnet has a rectangular or circular cross-section with a central axial bolt hole. A sintered iron "pole tip" or "shoe" with a center hole is mounted atop each magnet. A non-magnetic bolt is passed through the holes of the pole tip and magnet, being screwed into a threaded radial hole tapped into the flat machined mounting surface of the hub to hold one end face of the magnet against the machined surface. A known, but more costly variation of this mounting arrangement utilizes a shallow groove machined into each of the otherwise flat mounting surfaces and has the ends of the magnets partially constrained within these grooves. This arrangement causes the hub and its groove to engage and constrain three of the six faces of the typical rectangular magnets used with such rotors.
While this common rotor construction is effective and has been in use for many years, it has a number of well-known shortcomings.
First, the sintered iron pole tip is brittle, and being largely exposed, is easily broken if hit by another object. When installing a rotor within the stator of a motor or generator housing, the rotor can be suddenly and somewhat strongly pulled into the narrow, closely fitted rotor chamber of the stator by magnetic force, sometimes causing a bolt head or the pole tip itself to hit the stator housing and crack or damage the pole tip.
Second, there are difficulties in attaching the magnet to the hub. To prevent movement between the pole tips, magnets and the machined surfaces of the hub, adhesive has been used between these members. Effective use of an adhesive requires that the mating surfaces be scrupulously clean. It has also been found in practice that if the magnet or pole tip is bumped, the adhesive sometimes shatters and thereafter becomes ineffective.
Third, the bolts which attach the magnets and pole tips to the hub must be non-magnetic to prevent undesirable leakage of magnetic flux through the bolt hole and along the bolts. Thus, weaker and more expensive metals such as stainless steel must be used in the bolts instead of hardened steel. The rotor design must account for this lower strength, often by using thicker, heavier and more expensive bolts.
Fourth, the bolt hole formed in the magnet reduces the magnet's field strength and thus the net power production of the transducer is reduced.
Fifth, molding of the sintered pole tips with the central bolt hole requires a core in the mold and produces variable pole tip dimension tolerances for its bolt hole; assembly may require the use of shimming washers or other reshaping of the pole tip to attain proper alignment. Sintered materials cannot be easily machined, often requiring grinding as the alternative to shimming with washers. If the outer periphery of the pole tips are machined after assembly of the rotor to produce a uniform rotor-stator air gap, extreme care must be taken in the machining, using a very thin cut. Nevertheless, the sintered pole tips sometimes still break.
Sixth, the presence of the central hole in the sintered steel pole tip results in a loss of magnetic field strength and a resulting loss of rotor efficiency.
Seventh, the hub requires exacting machining to produce accurate and smooth surfaces for attachment of the magnets. The hub is expensive to manufacture, and a slight misalignment results in a hub which is unbalanced. Time consuming placement of shimming washers between the magnet and hub is sometimes required to bring the magnets into alignment. If not properly aligned, such misalignment during operation may lead to unwanted vibration, loosening of the bolts, breakage of pole tips and/or magnets, and, worst of all, catastrophic rotor failure and a required shutdown of the motor or generator for repairs. In some applications, such as uninterruptable power systems, the unplanned shutdown of a generator or crucial motor can be very costly to a user.
Eighth, forces exerted on the attachment bolts now used to secure the magnets and pole tips have a major longitudinal vector, tending to stretch the bolts and sometimes leading to further loosening of the magnets and pole tips.
Ninth, if an attachment bolt is over-torqued during installation, the brittle pole tip can be easily broken.
Tenth, the magnets are supported at only one end, and thus are subject to bending forces. During operation significant torque must be absorbed by the magnets, the pole tips and the attachment bolts, and this torque tends to sometimes loosen the bolts and cause eventual rotor failure.
While the above list of problems or disadvantages of the prior art rotor is not exhaustive, it suggests a need for improvement in rotor design.
One particular use of a smaller permanent magnet generator is as a "pilot exciter" for a large brushless field generator wherein the exciter operates off the same driveshaft. When a heavy starting load or short-circuit condition overloads the field generator, the exciter supplies constant voltage to the field generator's voltage regulator, forcing the field generator into saturation and supplying the necessary output current to handle the starting load or clear the fault condition, i.e. trip the circuit breaker.
In this generating application, it is extremely important to minimize the exciter downtime, since an excessive starting load or short-circuit condition which is not exciter-enhanced may result in shutdown of the field generator. Thus, it is desirable to eliminate the possibilities of rotor failure due to any of the reasons indicated above, and to develop a rotor with greater reliability using simpler construction techniques and at lower cost.